In semiconductor manufacture oxidation of silicon is important in forming oxides for field isolation structures and thin oxide layers for gates or other components of MOSFETS. One oxidation process that is used to separate the active regions on a silicon substrate is known as local oxidation of silicon (LOCOS). During a conventional LOCOS process, a pad oxide and a barrier layer of a material such as silicon nitride are formed on a silicon substrate. The barrier layer is then patterned and etched to expose the pad oxide or silicon substrate in certain areas. The silicon substrate is then subjected to thermal oxidation. By exposing the uncovered or exposed areas of the silicon substrate to a high temperature oxidizing atmosphere, a relatively thick field oxide (FOX) is grown only in the exposed areas. The barrier material is then removed and the substrate may then be processed further for forming the semiconductor devices.
FIGS. 1 and 2 illustrate a conventional LOCOS process. The process begins with a silicon substrate 10. A pad oxide layer 12 is formed on the substrate 10 by a suitable deposition or growth process. A silicon nitride layer 14 is deposited on the pad oxide layer 12. Typically the silicon nitride layer 14 is deposited using a low pressure chemical vapor deposition process (LPCVD) with a silane or dichlorosilane source. The silicon nitride layer 14 and pad oxide layer 12 are then patterned and etched using a photoresist mask 16. The exposed areas 18 of the substrate 10 become future field regions and the covered areas 20 of the substrate 10 become future active areas.
The substrate 10 is then thermally oxidized using an oxidizing atmosphere that includes pure oxygen and water vapor. Typically, thermal oxidation is performed in a vertical or horizontal tube furnace at a temperature of 800-1300 C. As shown in FIG. 2, this forms a field oxide (FOX) 22 over a field implant region 24 of the substrate 10. The field oxide 22 grows not only vertically in the exposed areas 18 (FIG. 1) but also laterally under the edges of the silicon nitride layer 14. This lateral oxide encroachment under the nitride layer 14 is known as the "birds beak" 26. In general, the birds beak 26 can grow to a thickness of about half the thickness of the field oxide 22.
The formation of the bird's beak 26 reduces the area of the moat region 28 available for active semiconductor devices. In addition, there is a field encroachment region 30 wherein the field implant can laterally diffuse into active areas. This encroachment tends to increase the threshold voltage (Vt) near the edges of the active devices, and the full device width is unavailable for the channel region.
In the past, various semiconductor manufacturing processes have been proposed to improve the conventional LOCOS process and to decrease the size of the bird's beak formed during the LOCOS process. These processes include: sealed interface LOCOS (SILO); side wall mask isolation (SWAMI); poly buffered LOCOS (PBL) and OSELO. One problem with these oxidation processes is that stress is produced in the oxidized layer causing crystal lattice dislocation loops. In addition, these processes require additional masking layers or oxidation steps.
In view of the foregoing, there is a need for an improved method for forming field isolation structures.
Accordingly, it is an object of the present invention to provide an improved method for forming field isolation structures using an ozone enhanced oxidation atmosphere and tapering.
It is another object of the present invention to provide an improved method for forming a field isolation structure having reduced stress and a sloped, tapered or faceted topography without detriment to adjacent semiconductor structures.
It is yet another object of the present invention to provide an improved field isolation structure formed by an ozone enhanced oxidation atmosphere and tapering.
It is a further object of the present invention to provide a method for forming a field isolation structure that is simple and adaptable to large scale semiconductor manufacture.
Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.